feat: add directory picker for OPDS downloads with per-server default path

When downloading a book via OPDS, a directory picker now lets the user
choose the save location instead of always saving to the SD root. Each
OPDS server has a configurable default download path (persisted in
opds.json) that the picker opens to. Falls back to "/" if the saved
path no longer exists on disk.

- Add DirectoryPickerActivity (browse-only directory view with "Save Here")
- Add PICKING_DIRECTORY state to OpdsBookBrowserActivity
- Add downloadPath field to OpdsServer with JSON serialization
- Add Download Path setting to OPDS server edit screen
- Extract sortFileList() to StringUtils for shared use
- Add i18n strings: STR_SAVE_HERE, STR_SELECT_FOLDER, STR_DOWNLOAD_PATH

Made-with: Cursor

src/activities/browser/OpdsBookBrowserActivity.h

@@ -17,12 +17,13 @@
 class OpdsBookBrowserActivity final : public ActivityWithSubactivity {
  public:
   enum class BrowserState {
-    CHECK_WIFI,      // Checking WiFi connection
-    WIFI_SELECTION,  // WiFi selection subactivity is active
-    LOADING,         // Fetching OPDS feed
-    BROWSING,        // Displaying entries (navigation or books)
-    DOWNLOADING,     // Downloading selected EPUB
-    ERROR            // Error state with message
+    CHECK_WIFI,          // Checking WiFi connection
+    WIFI_SELECTION,      // WiFi selection subactivity is active
+    LOADING,             // Fetching OPDS feed
+    BROWSING,            // Displaying entries (navigation or books)
+    PICKING_DIRECTORY,   // Directory picker subactivity is active
+    DOWNLOADING,         // Downloading selected EPUB
+    ERROR                // Error state with message
   };
 
   explicit OpdsBookBrowserActivity(GfxRenderer& renderer, MappedInputManager& mappedInput,
@@ -55,6 +56,11 @@ class OpdsBookBrowserActivity final : public ActivityWithSubactivity {
   void fetchFeed(const std::string& path);
   void navigateToEntry(const OpdsEntry& entry);
   void navigateBack();
-  void downloadBook(const OpdsEntry& book);
+  void launchDirectoryPicker(const OpdsEntry& book);
+  void onDirectorySelected(const std::string& directory);
+  void onDirectoryPickerCancelled();
+  void downloadBook(const OpdsEntry& book, const std::string& directory);
   bool preventAutoSleep() override { return true; }
+
+  OpdsEntry pendingBook;
 };